The use of ceramic or refractory compositions for electrical devices such as insulators and resistors has been well known for many years. For example, igniters for fluid fuel burning systems have been described in the Mikulec U.S. Pat. No. 3,372,305. Such igniters, which may be referred to as spiral igniters are composed of a non-metallic resistance material such as a very dense, recrystallized silicon carbide.
In the Mikulec spiral igniters, a pair of diametrically opposed slots are cut through the radial wall thickness of an elongated, hollow, tubular resistance body to form two semi-circular laterally spaced legs. A pair of closely spaced spiral slots are then cut through the wall of the body to form a pair of helical bands and an end connecting portion to provide a continuous electrical path. This type of igniter has had wide spread commercial success, but is relatively fragile and includes a number of manufacturing steps. In addition, the yield in the manufacturing process is often less than desired.
High strength refractory resistor compositions are disclosed in the U.S. Pat. No. 3,890,250 of Richerson. As disclosed therein, a resistor is composed of from 50-90% by weight of silicon nitride and 10 to 50% by weight of silicon carbide. The electrical resistivity varies from a maximum of 1.times.10.sup.7 ohm cm to about 0.002 ohm cm. The high strength characteristics are the result of hot-pressing the mixture of powders which brings about almost complete densification. However, when this material is used as an igniter, the hot zone degrades rather quickly e.g. goes from a resistance of about 180 to about 250 ohms and the cold ends from about 40 ohms to about 150 ohms.
A further approach to igniters is disclosed in the U.S. Pat. No. 4,205,363 of Maeda et. al. The igniter disclosed therein comprises about 95% silicon carbide and up to 5% of a negative doping agent such as nitrogen, phosphorous, arsenic, antimony and bismuth. However in order to obtain rapid heat up time as required for igniting a gas, the igniter must be made with a small cross section. Therefore, the igniters are very fragile.
A more recent igniter development is disclosed in Washburn, the U.S. Pat. No. 5,085,804. As disclosed therein, an electrical device is made up of a sintered, preferably a hot-pressed mixture of fine powders of aluminum nitride or silicon nitride, silicon carbide and molybdenum disilicide with all three present in substantial quantities. The total structure of the disclosed refractory body is essentially that of two separate, but intertwined structures with one structure being contained within the other structure. An essential feature of this type of total structure is that even though there is intimate contact between the two intertwined structures there is no or very little chemical reaction between or diffusion of atoms from one structure to the other.
Ceramic and refractory compositions have also been used in the manufacture of ceramic heaters which incorporate a ceramic substrate and a heat generating resistor disposed in the interior or on the surface of the substrate. As disclosed in the U.S. Pat. No. 4,804,823 of Okuda et al., a ceramic heater comprises a sintered silicon nitride body as a substrate, a resistance heater on the surface of the substrate and terminals connected to both ends of the heat-generating resistor. The heat-generating resistor is composed of a ceramic layer containing titanium nitride (TiN) or tungsten carbide (WC).
It is now believed that there may be a significant commercial demand for a composite monolithic element in accordance with the present invention. It is also believed that the elements in accordance with the present invention are particularly applicable as solid state igniters for fluid fuels, and are also suitable for many other applications such as heating elements, integrated circuits and other electrical devices which incorporate resistors and insulators. The demand for such elements is further enhanced by the advantages which are inherent in the composite monolithic elements.
The composite monolithic elements in accordance with the present invention have a seamless bond which is formed by short range diffusion of fine particles. Therefore, there is no strength limiting seam and the elements are less fragile than more conventional macro composite layered structures. This is an important consideration in the manufacture and use of hot surface igniters. Furthermore, the igniters in accordance with the present invention incorporate a relatively robust substrate which fully supports the more fragile resistor and protects the resistor from breakage during handling and further manufacturing steps such as the testing, installation of a shield, installation in an appliance and use.
In addition, composite monolithic elements containing saturation doping in accordance with the present invention can be manufactured with consistent, reproducible electrical resistivity and constant dopant concentrations. Such elements or igniters have also been found to have consistent strength, thermal expansion and thermal shock effects, and resistance to in-use degradation through oxidation and dopant diffusion.
Furthermore, the manufacturing process in accordance with the present invention is effective in providing green bodies with relatively high particle concentrations and very high density, fine grain bodies under less severe conditions than normally encountered with similar products. This use of high green density results in relatively little shrinkage and deformation during the removal of organic binders, minimal shrinkage and deformation during sintering, and essentially no shrinkage during reaction bonding. The process is also effective in obtaining relatively high yields of products with consistent chemical, mechanical and electrical properties.